The present invention relates to a method of purifying exhaust gas of an internal combustion engine particularly, the present invention relates to a method of purifying exhaust gas by controlling the amount of secondary air supplied into an exhaust system of the engine so as to stabilize the purification efficiency of a catalytic converter.
In a system for purifying exhaust gas, which system simultaneously reduces the noxious components of HC, CO and NOx contained in the exhaust gas by means of a three-way catalytic converter provided in an exhaust system of an internal combustion engine, it is necessary to control the air-fuel ratio of the exhaust gas which flows into the three-way catalytic converter within a very narrow range (hereinafter referred to as "window range") around a stoichiometric air-fuel ratio. This is because the purification efficiency of the three-way catalytic converter becomes highest in the above-mentioned window range. In the conventional system of purifying exhaust gas by controlling the amount of the secondary air which is supplied into the exhaust system, the air-fuel mixture which is supplied into the combustion chambers of the engine is set to be rich in comparison with the stoichiometric air-fuel ratio. Then, a suitable amount of secondary air is supplied into the exhaust system upstream of the catalytic converter, thereby controlling the exhaust gas air-fuel ratio within the above-mentioned window range. The amount of the supplied secondary air, in this case, is controlled so as to gradually increase when an air-fuel ratio sensor, which is provided in the exhaust system of the engine and detects the concentration of a particular component, such as an oxygen component, contained in the exhaust gas, detects that the exhaust gas air-fuel ratio is rich; it is controlled so as to gradually decreases when the air-fuel ratio sensor detects that the exhaust gas air-fuel ratio is lean.
However, in such a system as mentioned above, if the average level of the air-fuel ratio which is adjusted in an intake system (hereinafter referred to as "base air-fuel ratio") deviates from a standard value due to change in the driving condition of the engine or to deterioration of parts, the average air-fuel ratio of the exhaust gas deviates from the stoichiometric air-fuel ratio, thereby lowering the accuracy of the control of the air-fuel ratio as a whole. As a result, the purification efficiency of the three-way catalytic converter is reduced, thereby causing bad emissions.